1. Field of the Invention
The present invention relates to a differential data transmitter, and more particularly to an improvement of a pre-emphasis function in the output section of the transmitter. For example, the differential data transmitter is used in a high-speed serial data transmission system, such as a router that handles data of about 3.2 Gbps, or a mount board that performs data transmission between LSI chips.
2. Description of the Related Art
In a high-speed serial data transmission system, it is preferable to ensure a sufficient eye pattern (the eye pattern is an opening observed at the center when overlapping signal waveforms) in a differential data signal received by a receiver on an opposite-party side. For this reason, a technique called “pre-emphasis” is applied to the output section of the transmitter on a sender side. Pre-emphasis means that signal frequency components to be attenuated are emphasized in advance by the data driver of the transmitter on the sender side. The signal frequency components are attenuated, according to the characteristics of the semiconductor device package on the sender side and the data transmission lines. With pre-emphasis, it is possible to expand the signal amplitude, thereby ensuring a sufficient eye pattern, in a differential data signal received by the receiver on the opposite-party side.
FIG. 9 is a block diagram showing the basic arrangement of the output section of a conventional differential data transmitter employed in a high-speed serial data transmission system.
This output section of a transmitter includes two pre-drivers 61 and 62 that gradually increase driving current values, a delay circuit 60, and an output driver 63 for large-current driving. The output driver 63 generates a pre-emphasis waveform signal from the output signals of the two pre-drivers 61 and 62. The output driver 63 is formed of a subtraction circuit that performs subtraction on two input signals to generate a pre-emphasis waveform signal.
In the output section of a differential data transmitter shown in FIG. 9, the serial transmission path that transmits a differential input signal (consisting of positive and negative signals) is divided into two paths STP61 and STP62 extending in parallel with each other. One STP61 of the paths is a first path (main signal path) for transmitting the data signal (serial differential signal) to the subsequent stage as it is. The other path STP62 is a second path (emphasis signal path) for transmitting a signal to be used for emphasizing the data signal (serial differential signal).
Through the first path STP61, the data signal is inputted into the output driver 63, while it is buffered and caused by the first pre-driver 61 to gradually increase its current value. In the second path STP62, the data signal is provided with a delay of a certain time by the delay circuit 60. Then, the data signal is inputted into the output driver 63, while it is buffered and caused by the second pre-driver 62 to gradually increase its current value.
Accordingly, the data signal transmitted through the second path STP62 used as a signal for emphasizing data is inputted into the output driver 63 with a delay time given by the delay circuit 60, as compared to the data signal transmitted through the first path STP61. The output driver 63 receives the output signals of the two pre-drivers 61 and 62, and performs subtraction on the two data signals to generate a differential output signal having a pre-emphasis waveform.
FIG. 10 is a signal waveform chart used for explaining the concept of pre-emphasis by subtraction, where the output driver 63 is formed of a two-input subtraction circuit.
In FIG. 10, Y1 denotes a current for driving a first differential circuit for the main signal in the output driver 63, and Y2 denotes a current for driving a second differential circuit for the emphasis signal in the output driver 63. In a period of a delay time T_Delay that begins from a change point of the data, the output currents of the two differential circuits act to reinforce each other, thereby driving with a current of Y1+Y2. By contrast, in a period until the next change point of the data after the delay time T_Delay, the output currents of the two differential circuits act to cancel each other, thereby driving with a current of Y1−Y2.
In this case, the delay time T_Delay given to the data signal (emphasis signal) transmitted through the second path STP62 does not only detect change points of data, but also determine a data pre-emphasis period.
FIGS. 11A and 11B are schematic views respectively showing conventional examples of the delay circuit 60 shown in FIG. 9.
FIG. 11A shows a buffer line on which a plurality of buffer circuits or inversion circuits are connected in series. This buffer line uses the buffer circuit or inversion circuit as delay elements connected in series to utilize propagation delay obtained by the delay elements. In this case, the number of delay elements is adjusted to set a delay time. The delay time, however, depends on the gate pattern corners of transistors used, temperature, power supply voltage, and so forth.
FIG. 11B shows a D-type flip-flop circuit (D-FF) driven by a clock signal CLK. The clock signal CLK used for the D-FF has the same frequency as the data rate of an input signal. The D-FF holds a signal for a period of time corresponding to the cycle of the clock signal CLK or a half thereof to provide the signal with a delay.
In either of the circuits shown in FIGS. 11A and 11B, the delay time T_Delay depends on the circuit arrangement, and is predetermined when designed. In other words, the data pre-emphasis period is fixed when designed. However, the optimum value of the data pre-emphasis period varies, depending on various conditions, such as the length of transmission lines, the external environment, and so forth. In this respect, since the conventional differential data transmitter has an output section in which the delay time is predetermined when designed, it is difficult to realize an optimum pre-emphasis period.
For this reason, a high-speed serial data transmission system should be provided with a differential data transmitter having an output section that can realize an optimum pre-emphasis period, depending on various conditions, such as the length of transmission lines, the external environment, and so forth, so as to expand signal amplitude on an opposite-party receiver side.